An insulating coating applied to an electrical steel sheet used for motors, transformers and the like is required to have various properties such as not only interlaminar resistance but also convenience during processing and molding, stability during storage and use, and so on. Particularly an insulating coating with excellent punchability can reduce frequency of replacement of press mold during punching. Since electrical steel sheets are used in a variety of applications, it is attempted to develop various insulating coatings depending upon the application. When an electrical steel sheet is subjected to punching, shearing, bending, or the like, magnetic properties are deteriorated due to residual strain, so that stress relief annealing at a temperature of about 700° C. to 800° C. is frequently conducted for solving this problem. In this case, therefore, the insulating coating must be resistant to the stress relief annealing.
Insulation coatings applied to electrical steel sheets are roughly classified into three types:
(1) Inorganic coating placing great importance on weldability and heat resistance, and being resistant to stress relief annealing;
(2) Resin-containing inorganic coating aimed at achieving both punchability and weldability, and achieving resistance to stress relief annealing (i.e., semi-organic coating); and
(3) Organic coating being unable to be subjected to stress relief annealing in a special application.
However, only the coatings containing an inorganic component of types (1) and (2) above are resistant to the stress relief annealing as general-purpose product, and both of them have been used to normally contain a chromium compound. Particularly, the chromium insulating coating of type (2), which is manufactured by one coat one bake, can remarkably improve punchability comparing with inorganic insulating coatings, and therefore is widely used.
However, as environmental awareness is rising in recent years, chromate free products having an insulation coating free of chromium compound are demanded by consumers even in the field of electrical steel sheets. The following are techniques for forming insulating coatings of type (2) by applying a surface coating solution to the surface of an electrical steel sheet, where the surface coating solution contains both an organic component and an inorganic component, and does not contain any chromium compound.
JPH10-130858A (PTL 1) describes an electrical steel sheet with an insulating coating that can be manufactured by low-temperature baking and subjection to stress relief annealing, being excellent in boiling water vapor resistance and solvent resistance. The insulating coating contains a resin that has a glass transition point of 30° C. to 150° C., and alumina-containing silica.
JPH10-46350A (PTL 2) describes a technique for forming an insulating coating using a water solution as surface coating solution, where the insulating coating is excellent in properties such as corrosion resistance before stress relief annealing, and the water solution is prepared by adding an organic substance consisting of one or more water-soluble or emulsion type resins to an inorganic colloidal substance consisting of one or more of colloidal silica, alumina sol, and zirconia sol.
JP2007-197820A (PTL 3) describes an electrical steel sheet excellent in corrosion resistance, adhesion property, solvent resistance, and anti-sticking property, comprising an insulating coating consisting of a polysiloxane polymer obtained by copolymerizing polysiloxane and various organic resins, and inorganic compounds such as silica and silicate.